1. Field of the Invention
The present invention relates to a frictional engagement apparatus using a wet frictional material.
2. Description of the Related Art
FIG. 1 shows an example of a frictional engagement apparatus in which torque is transmitted through contact between driving plates 2 fitted in a spline portion 51 of a hub 5 inserted onto an input shaft 6 and driven plates 1 fitted in a spline portion 41 of a retainer 4. In the drawing, the reference numeral 3 designates a pressure plate and the reference numeral 7 designates a piston for pressing.
Presently, in view of energy and environmental problems, such a frictional engagement apparatus is required to be small in size, light in weight, small in operational shock, and high in torque capacity. The foregoing requirements are extremely strong.
In a conventional frictional engagement apparatus, however, lubricating oil with a low friction coefficient has been widely used in order to reduce operational shock, and, therefore, miniaturization of the frictional engagement apparatus inevitably causes low torque capacity. Accordingly, the operational pressing force is required to be made high to increase the torque capacity, resulting in defects relating to reduction of the separation life of a wet frictional material, generation of heat spots and thermal deformation in a mate-frictional surface (driven plate), increase in size of an oil pump, reduction in endurance life such as leakage of operational oil, and the like. It is therefore necessary to take the safety factor into consideration in order to solve the above problems and it is difficult to reduce the size of the apparatus per se.
In order to solve the foregoing problems, a high dynamic friction coefficient (.mu.d) and a high static friction coefficient (.mu.s) could be indeed obtained by forming a single layer of a ceramics thin film on the mate-frictional surface, and the above problems such as miniaturization of the apparatus could be solved by maintaining an ideal torque waveform without any change over time.
In the present and future environment, however, the frictional engagement apparatus is being used and will be used under stricter and stricter conditions, and a film formed on the frictional surface according to conventional techniques is affected by acid components and the like generated by decomposition of lubricating oil due to extremely high exothermicity, surface pressure (force), and high heat so that cracks, separation, or falling off is caused, and thereby making it difficult to maintain stable quality.
If a boron nitride group film containing cubic boron nitride (C-BN), cubic SiBN (C-SiBN) or cubic BCN (C-BCN), which is a super hard material, is formed as a matrix on the frictional surface, a brittle layer is formed by dispersion of boron (metal boron) in the matrix so that the matrix becomes brittle disadvantageously.
Moreover, such ceramics thin film materials are so expensive that a metal plate subjected to surface treatment is more expensive than a current or conventional metal plate, and it is difficult to improve the function correspondingly to the increase of the cost. Accordingly, it has remained difficult to succeed in manufacturing these ceramic plates since the increase in performance does not justify the increase in price.
Further, FIG. 2 shows a conventional arrangement of driven and driving plates 1, 2 and FIG. 3 shows a side view thereof. As shown in FIGS. 2 and 3, each driving plate 2 includes a metal plate 9 upon each side surface of which is bonded a frictional material 8. Each driven plate 1 has a metal plate 9, opposite side surfaces of which have a ceramics thin film formed thereon. In the drawings, the reference numeral 91 designates spline teeth provided by the metal plate 9 for both the driven and driving plates 1, 2 which is a matrix, for the sake of gearing with a spline. However, such a configuration of a frictional material 8 bonded on each of the opposite surfaces of the driving plate 2 causes defects which will be described later.